Max. photos per second
In still image (photo) mode, all pixels have to be read and this takes 200mS. So 'in theory' we can achieve 5 fps (frames per second) - but what's the actual maximium ?
Preventing Preview
The no preview flag (-n) doesn't actually 'stop' previews - instead it just sends the preview data to DEV NUL. However this does 'save some time' (since normally the data would have to be directed to the display memory).
A Previews is a short video sequence. For videos, something called 'rolling shutter mode' is used, whilst for still photo capture the sensor is operated in a different mode.
This means the camera has to be switched from Preview (video) mode to still mode before a photo can be taken - and this not only takes time but also corrupts the first 'frame' read in still capture mode. Even worse, at the then of each photo 'by default' the camera drive switches back to preview mode !
To avoid all the time (and 'first frames') lost switching modes, a new flag was intruduced called 'burst mode'. When the -bm flag is used, the camera does not re-enter Preview after the photo has been taken.
Note that the camera has to start in Preview mode because that's when it's setting is Automatic Gain Control (AGC). The AGC value starts at zero and is 'wound up' until image data is found (0 means 'totally underxposed' i.e. black frame). It's possible to 'stop' the AGC (which freezes it at whatever value it's reached) however that's usually 'not a good idea' :-)
Setting the -tl time
You might think that setting -tl to 200 (200 mS = 5 fps) would get you the maximium possible number of frames. Well 'yes', but if the rest of the system 'can't keep up' (convert to jpg, write to SDHC etc) you will end up reading frames 'late' ... and once the first is late the problem is likley to 'cascade' until you end up skipping frames.
So, for max. speed, you set -tl to 0 (which means the camera runs for the -t time generating photos as fast as the rest of the system asks for)
JPEG or YUV ?
The first thing 'output' from the camera is YUV - this then has to be processed into whatever final output format you ask for. Some formats - such as jpeg - are processed by the GPU, so take 'almost no' extra time.
Even so, YUV will be avialble faster than jpeg. The 'problem' with choosing YUV is that the image size will be 7.5Mb - and it's slower to strore 7.5Mb than it is to convert to jpeg and stor the (typical) 2.2 Mb result.
Dropping the thumbnail
By default, a thumbnail (-th w:h:q) is added to jpeg photos. This takes time (and uses extra space), so setting '--thumb none' (or '-th 0:0:0') speeds up the jpeg photo output.
Writing to RAM disk
There does not appear to be much difference between writing to a good Class 10 SDHC card or writing to 'tmpfs' RAM disk - however, ever little bit counts